In a large deep skillet or pot, heat 2 tbsp of oil over medium-high heat. Add mushrooms with a generous pinch of salt. Cook, stirring every 1-2 min until they are browned and any liquid has evaporated, about 10 min. Remove the mushrooms from the skillet and set aside.
In the same skillet, add the remaining 1 tbsp of oil. Add the onions and cook over medium heat. Stir occasionally until the onions are soft and golden brown bits begin to form on the bottom of the pan. Deglaze with water and season with salt. Continue cooking and deglazing with water until caramelized, which can take about 20-25 min. When they’re almost done, add balsamic and stir to combine. Cook 2-3 min until acidity softens. Taste and adjust if necessary.
Adjust heat to low. Add the mushrooms back with minced garlic and sun-dried tomatoes. Sauté for 2 min until fragrant. Pour in the chicken stock and add the tortellini. Bring to a simmer and cover. Cook according to package. If needed, add more stock until the tortellini is plump and fully cooked. Each brand of tortellini requires a different amount of liquid so follow your instinct.
Once tortellini is cooked, pour in half the heavy cream and grated Parmesan cheese. Stir the mixture gently over low heat until the sauce thickens and the cheese is melted, about 2-3 minutes. If you’d like the sauce creamier, add more cream until satisfied.
Fold in spinach & cook just until it wilts. Taste for last minute seasoning. Transfer to a plate & top with parm, black pepper & fresh thyme.